If you look they are marked R (Regional) and REX (regional Express)

For most of Europe these (and other similar designations such as the more common RE or TER in France) will be trains where reservations are not offered and depending on country/operator tickets can also not be available online.

Just add to your pass or travel diary before boarding and that is all you need.

Edit: The bottom train on that list is NJ (Nightjet) this is a long distance night train with limited seating nearing the end of it’s journey, these are not usually open for local traffic so ignore these.

Oh I see! Thanks! But how come when I look the trip on interrail website it says reservation is mandatory? 

It doesn’t on the interrail app and it shouldn’t anywhere else.